1. Field of Invention
The present invention relates to a pumping unit for actuating a down hole pump for raising water or oil held in an earth formation to the surface. More particularly, the present invention relates to pumping units which include a samson post pivotally connected by a center bearing to a walking beam. One end of the walking beam has fixedly connected thereto a stationary counterweight to counter balance the other end of the walking beam, to which other end is connected a string of pump rod operatively connected to the down hole pump.
2. Description of the Prior Art
Walking beam or "horsehead" type pumping units have been used for decades to actuate down hole pumps, forcing water or oil to the surface from a subterranean water or oil bearing formation. All of the horsehead pumping units include a wire line or cable connected to the horsehead, passing over a convex outer face curved about the pivotal axis of the walking beam. The cable or wireline lies on the curved outer face of the horsehead tangent to the outer face in a vertical position over a well head regardless of the position of he horsehead in its stroke. The wireline connects at its lower end to a drill string of pump rod by wire clamps or connecting devices. The flexible cable or wireline lying on the curved face of the horsehead provides for vertical reciprocation of the pump rod as the horsehead goes through its stroke, without bending the rod. The pump rods are connected to the piston of the down hole pump so that, as the horsehead moves upwardly during a work stroke of an oscillatory cycle, fluid is pumped. During a return stroke of the oscillatory cycle the piston of the down hole pump is returned to its starting position.
In all of the known walking beam pumping units some counterbalancing arrangement is used. The oldest known walking beam pumping units, conventional units, have a prime mover and the samson post both connected to the base. In the conventional pumping unit, a counterweighed crank is connected to the prime mover through a gear reducer. The crank in turn is connected at the counterweight through a pair of pitman rods to one end of the walking beam. As the counterweight realizes its potential energy and passes from the top to the bottom of its rotation, it assists the prime mover in overcoming the load of the pump rods and fluid being pumped during the work stroke by counterbalancing the load. During the return stroke, the prime mover must raise the counterweight from a bottom position to a top position so that the potential of the counterweights can again be realized during the next work stroke. This is a simple system but requires a relatively high horsepower rated prime mover to move the counterweight through the rotational cycle.
A relatively recent innovation to pumping units pivotally connects the samson post at the one end of the walking beam while the pitman rods are connected through a cross-yoke equalizer bearing to the other or horsehead end of the walking beam. A crank includes counterweights, but the pitman rods are connected to rearward extensions of the crank, rather than connected at the counterweight, as in the conventional units.
Beam balanced pumping units have no counterweight associated with the crank, but instead use a fixed counterweight mounted to the one end of the walking beam opposite the horsehead to offset some of the down hole loads. Such a beam balanced pumping units is seen in E. Patterson, U.S. Pat. No. 1,986,012. As in all known beam balanced pumping units, the counterweights of Patterson are used to counterbalance the string of pump rods, the work output of the prime move being used to reciprocate the walking beam and pump the fluid. Patterson is unique over other prior art pumping units in that the samson post carries the prime mover which turns a pair of sprockets mounted on the walking beam. A pair of pitman rods connect the sprockets to the base so that the oscillating up and down movement of the horsehead required in such pumping units is obtained by pivotally connecting the pitman rods to the base, rather than to the walking beam, as in the conventional pumping units. Patterson, along with other prior art structures, shows and discloses a structure consisting of various different openings in the sprocket through which pin connections are secured. Variations of which opening is used, varies the stroke length or distance through which the horsehead moves.
Instead of counterweights, an air tank carrying a piston rod is used to counterbalance some of the down hole weight of the pump rod and fluid during the work stroke in another variation of the walking beam pumping unit.
G. King, U.S. Pat. No. 4,306,463, shows a walking beam driven in the oscillatory up and down pumping cycle by a pair of periodically moving balance weights, interconnected for concurrent periodic movement. While one balance weight is falling to move the walking beam, the other weight is being raised by the prime mover, in this case an hydraulic cyclinder. The walking beam also includes stationary counterweights which serve to counterbalance the horsehead, pump rod load and maximum expected fluid column load. The moving balance weights do not rotate on the walking beam.
Various adjustable stroke mechanisms are shown for pumping units. T. Hoh, et al., U.S. Pat. No. 4,505,162 shows a slot formed in a crank arm. A wrist pin assembly is slideable in the slot and lockable at any point along the slot. A pitman rod is connected to the wrist pin. A. dingfelder, U.S. Pat. No. 4,502,343, shows a single pitman rod having an infinite adjustment between limits. A turnbuckle is used to change the length of the pitman rod. Adjustment of the connection point of the pitman rod to the walking beam is also shown. C. Johnson, U.S. Pat. No. 2,958,237, shows another threadable connection for varying the stroke length of a walking beam pumping unit.
None of the prior art structures or prior art patent publications show use of a static counterweight connected to the beam to balance the pump rod load and a dynamic rotating counterweight mounted on the beam to balance the cyclical fluid column load. The advantage of such an arrangement, such as the invention to be described hereinafter, includes lowering the torque and horsepower requirements of the prime mover required for a given pumping unit. An additional benefit of such a rotating counterweight, as presently contemplated, is reduction of the stress in the pump rod string by minimizing the difference between the peak pump rod load and the minimum pump rod load.